A souterrain, to use the term archaeologists have borrowed from the French for "underground way", is a man-made tunnel or chamber, typically constructed from drystone walling and roofed with large capstones, then buried beneath the earth. They are associated almost exclusively with early medieval settlement in Ireland, dating broadly from around the sixth to the twelfth centuries, and are found in their greatest concentrations in the north and west of the country.

What souterrains were actually for remains a matter of some debate. Cold storage is the most practical explanation, and the consistent underground temperature would certainly have suited the preservation of dairy produce, which formed a significant part of the early medieval Irish diet. Refuge is another theory, given that some passages are deliberately narrowed at intervals, forcing an intruder to slow down and making defence easier. In some cases they connect directly to ringforts, the enclosed farmsteads of the period, suggesting they formed part of a wider domestic and defensive complex. The souterrain at Graigue sits within this broader tradition, though the particular details of its construction, dimensions, and condition remain part of the undigitised record.